In these days, products in a market of electronic devices such as portable information devices and the like have been required to be compact and light-weight, which correspondingly increases the demand for forming circuit boards constituting the electronic devices to be compact and light-weight as well. Under these circumstances, package products such as BGAs reduced in package size by setting electrodes to rear faces of electronic components, CSPs of a nearly equal size to a semiconductor bare chip by further miniaturizing the BGA, and the like, and moreover, flip chip mounting for directly mounting onto a circuit board via bump electrodes of a semiconductor bare chip without forming a package are starting to be widely adopted.
However, the above flip chip electronic components such as BGAs, CSPs or the like have electrode connection parts set to the underside which cannot be seen from outside, thus hindering visual inspection to the connected parts, unlike conventional electronic components with leads. Nondestructive inspection using X-rays or the like should therefore be realized.
There is a method for inspecting whether or not electrodes of an electronic component and of a circuit board having the component already mounted are connected well with the use of a conventional transmission type X-ray inspecting machine to meet the above demand, in which X-rays passing the circuit board and the electronic component on the circuit board were converted to an image by an X-ray generator arranged to apply X-rays perpendicularly to the circuit board with the component mounted and by an X-ray detector for detecting the X-rays passing the circuit board, whereby the image was visually inspected or automatically inspected by an image recognition device. A positional deviation of a connected part, a short circuit of electrodes, excess/deficiency and voids of a connecting material such as solder or the like, scattering of solder balls, contamination by a foreign substance, etc. have been inspected in this manner.
In general, heavy metal materials such as lead, tin or the like of a high index of absorption to the X-rays are used as the connecting material for the electronic component and circuit board. The picked connecting material in the X-ray image is black, and therefore can be distinguished from a surrounding part of the connecting material. Since it is necessary to obtain a thickness of the connecting material such as solder or the like in three dimensions particularly for inspecting excess/deficiency of the connecting material, and since a quantity of the X-rays passing a substance decreases like an exponential function with respect to a thickness of the substance, a device or technique to obtain a relationship of the thickness of the object to be inspected and a density of the X-ray image has been needed.
In the related art, however, there is a problem that the X-ray image of a thin object exceeds a dynamic range of an image density and is saturated when a storage time for the X-ray is secured long enough to meet a thick object, as is clear from FIG. 12, due to the characteristic that the quantity of X-rays passing the substance decreases exponentially to the thickness of the substance and due to limits on the dynamic range of the image density and on a resolution of a pickup system. On the contrary, when the storage time for X-rays is secured short enough to match the thin object, there is a problem that the X-ray image of the thick object becomes a minimum density signal or smaller and cannot be measured.
When a double face-mounted circuit board is to be inspected by the above transmission type X-ray apparatus, components of a front face and a rear face of the circuit board are picked up overlapping, and hence hard to inspect by the ordinary pickup method and recognition method heretofore.
There is another method using X-rays for inspecting the double face-mounted board with components mounted to both faces of the board, whereby an X-ray generator disposed to apply X-rays slantwise to the circuit board with the components mounted, and an X-ray detector for detecting X-rays passing the circuit board are synchronously rotated on a plane parallel to the board, thereby obtaining a horizontal sectional image of the board with adjusting a focal point to a fixed height of the board while blurring the other faces in heights different from the fixed height by the rotary motion. Furthermore, an X-ray inspection method using a technique called X-ray laminography whereby the front side and rear side of the double face-mounted board can be inspected separately is actually put in use. However, the method is complicated in mechanism and thus expensive, with problems yet to be solved in terms of cost for the method to be practiced in mass production factories, although it can be executed at a laboratory level.
To solve the problems, an X-ray inspection method using an image differential which is simple in structure is being examined as a method for inspecting the double face-mounted board. According to the method, as shown in FIGS. 27 and 28, an X-ray image in a state with a component mounted only on one face (A face) is stored beforehand. After a component is mounted to a rear side (B face) of the board, the stored X-ray image of the one face (A face)-mounted board is subtracted from an X-ray image of the board of a state with the components mounted to both faces, so that an image of only the rear face (B face) is obtained.
The technique using the image differential as above necessitates not only storing the X-ray image of a large capacity of the one face-mounted board, but making sure that the one face-mounted board is identical to the board after the double-sided mounting, that is, requires holding images of the boards in a perfect one-to-one correspondence.
Even when images of the same board are matched in a one-to-one relationship, the board deflects during soldering with the use of a reflow or the like, bringing about a deviation between images. A perfect image of, e.g., only the rear face is not always obtained in spite of the subtraction to the transmission images, and noises are included. In other words, a positional deviation is generated between the X-ray image of the double face-mounted board and the X-ray image of the one face-mounted board. The positional deviation remains as noises hinder a correct judgment on the inspection. The X-ray inspection method using the image differential has not reached a practical level.
The present invention is devised to solve the aforementioned problems, and has for its object to provide an apparatus and a method for inspecting connection whereby an inspection accuracy for connected parts of mounted components can be improved in comparison with the related art, and a recording medium for recording programs executing the connection inspecting method.